[foreign-memaccess+abi] RFR: 8317324: Add an overload to bind a memory segment var handle at an offset

[Maurizio Cimadamore]

This PR adds an overload which binds a memory segment var handle offset coordinate to a provided constant value.

This is useful when the client knows that the accessed offset will be e.g. zero, or some other known constant value, thus reducing the number of coordinates that var handles clients will have to provide on each access.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/900/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=900&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317324
  Stats: 25 lines in 3 files changed: 21 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/900.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/900/head:pull/900

PR: https://git.openjdk.org/panama-foreign/pull/900